1. Field of the Invention
This invention relates generally to the field of logging while drilling tools. In particular it relates to such tools for measurement of formation characteristics such as bulk density, photoelectric effect (PEF), neutron porosity and borehole caliper by means of ultrasonic measurements. Still more particularly, the invention relates to apparatus and methods for making such measurements as a function of angular position about the borehole as the tool is turning in such borehole during drilling.
2. Description of the Related Art
U.S. Pat. No. 5,091,644 of Minette describes a method for analyzing formation data with a logging while drilling tool. Such patent describes dividing the cross section of the borehole into two or more sectors. Gamma ray density signals are divided into four quadrants: top, bottom, right and left for operations in deviated boreholes. The gamma ray signals are collected as to their energy level so as to produce energy spectra for each quadrant. The '644 patent indicates that long and short spaced detectors are used to collect gamma ray count rate data to produce compensated density measurements.
Each quadrant measurement is combined with the other, either as a simple average or as a weighted average to produce a density value characteristic of the formation. If the borehole has minimal washout, all four compensated density measurements are used. If there is extensive washout, the "bottom and the two side measurements" are used to calculate density of the formation. If the borehole suffers extreme washout, only the "bottom" measurement is used.
The '644 patent describes error minimization whereby the spine and ribs correction is obtained for long and short spaced detector gamma ray spectra and an analysis made from quadrant to quadrant so as to minimize rib error. One or more quadrants are selected so as to minimize the error in arriving at a density value characteristic of the formation.
The '644 patent suggests that the borehole be broken into four quadrants, bottom, right, top and left. It suggests that such measurements can be made from measurements in the tool itself or from information supplied via a communication bus from another tool. It suggests that information from an accelerometer or a magnetometer that is sent to the density tool is sufficient to break such borehole into four quadrants.
The '644 patent also discloses providing an acoustic caliper in alignment with a density source and detector for determination of standoff in from of the detectors at any given time. Such standoff information is used to minimize error of the density characterization of the formation due to standoff. It is also used to determine cross-sectional divisions of the borehole.
The disclosure of the '644 patent fails to identify a method to accurately determine a bottom contact point of a logging while drilling tool operating in a deviated borehole so as to accurately have information as to where the bottom of the borehole is as the sensors of the tool turn in the borehole.